Secondary base metal minerals occur mainly where the original sulphide ore body has been converted by chemical weathering to a partially or substantially oxidised form. Such deposits are mineralogically complex and may comprise a host of secondary minerals.
In the case of copper, relevant minerals include native copper (Cu), malachite (Cu.sub.2 (OH).sub.2 CO.sub.3), brochantite (Cu.sub.4 (OH).sub.6 SO.sub.4), antlerite (Cu.sub.3 (OH).sub.4 SO.sub.4), atacamite (Cu.sub.2 (OH).sub.3 Cl), azurite (Cu.sub.3 (OH).sub.2 (CO.sub.3).sub.2), chrysocolla (CuSiO.sub.3.2H.sub.2 O), cuprite (Cu.sub.2 O) and tenorite (CuO) as reflected for example in Hiskey, "In Situ Leaching Recovery of Copper--What's Next?", Biomine '94 Conference, Perth, Western Australia. These minerals may occur in association with gangue minerals such as silicates, e.g. quartz, iron oxides, e.g. goethite and haematite. The secondary minerals may also be associated with high value sulphide minerals, such as copper sulphides, being for example, chalcocite (Cu.sub.2 S) and covellite (CuS) or substituted copper sulphides, for example, chalcopyrite (CuFeS.sub.2), bornite (Cu.sub.5 FeS.sub.4), enargite (Cu.sub.3 AsS.sub.4), tetrahedrite (Cu.sub.12 As.sub.4 S.sub.13) and tennantite (Cu.sub.12 As.sub.4 S.sub.13). In addition, free or mineralised precious metals such as silver or gold may be present. As the mineralogy of the host orebody is complex, the metallurgy of separation of the various constituents of the host orebody has been correspondingly complex.
In the case of lead, minerals which may be included are cerussite (PbCO.sub.3), pyromorphite (Pb.sub.5 Cl(PO4)3), and the complex lead-vanadium minerals of South West Africa, the latter as discussed at length by Fleming in the paper "Effects of Soluble sulphide in the Flotation of Secondary Lead Minerals", Recent Developments in Mineral Dressing, Institute of Mining and Metallurgy (1953), p521 et seq. Specific minerals of interest may include vanadinite 3Pb.sub.3 (VO.sub.4).sub.2 PbCl.sub.2) and descloizite (PB,Zn).sub.3 (VO.sub.4)PbZn(OH).sub.2 !. In the case of zinc and lead, an important source of secondary minerals includes the ferritic residues left after acid leaching of roasted zinc sulphide concentrates. Such residues are rich in lead and other base metals in the form, for example, of zinc ferrite and plumbojarosite or lead sulphate (anglesite) and may grade up to 20% lead when concentrated by flotation.
Zinc minerals of interest may include smithsonite (ZnCO.sub.3), willemite (Zn.sub.2 SiO.sub.4) and zincite (ZnO).
Partially oxidised minerals may also be encountered at mine sites where freshly mined sulphide ore may oxidise and separation behaviour be adversely affected. For example, chalcopyrite and chalcocite may at least partially oxidise following mining.
U.S. Pat. No. 3,883,421 (Cutting et al) discusses a process wherein a sulphidising agent is added to a pulp of an ore containing secondary copper minerals. The sulphidising agent may, for example, be a water soluble sulphide such as sodium sulphide or bisulphide. The sulphidising agent reacts with sulphidisable components, for example oxide minerals, in the pulp thereby forming a thin layer of sulphide on grains of the oxide minerals which are susceptible to collectors. This enables flotation of the sulphidised oxide minerals. Avoidance of excessive addition of sulphidising agent, that is, a quantity greater than stoichiometrically required, is attempted by controlling the addition of the sulphidising agent in accordance with a predetermined optimal range of oxidation-reduction potential for each specific ore type.
U.S. Pat. No. 4,011,072 (Holman et al) discloses a copper recovery process in which the EMF of a pulp of oxidised copper minerals as well as sulphide minerals is monitored. Water soluble sulphide is added to the pulp whenever and for so long as the pulp EMF is above about -30 mV with reference to a standard silver-silver chloride electrode. Reagent addition is discontinued whenever the EMF is less than about -30 mV. Following conditioning, the pulp is subjected to a froth flotation operation in the presence of a collector for copper sulphide minerals to produce a bulk concentrate containing most of the sulphide minerals and a substantial part of the oxidised minerals of the ore.
Jones et al., "Evaluation of Ion-Selective Electrode for Control of Sodium Sulphide Additions during Laboratory Flotation of Oxidised Ores", Trans. IMM, Part C, pp C99-C105 also relates optimal addition of sulphidising agent to appropriate electrochemistry and proposes a modern sulphide-ion selective electrode as the most promising method for measurement and control of sulphide concentrations in a pulp.
U.S. Pat. No. 4,585,549 (Malghan) discusses a further process for recovery of oxidised copper sulphide minerals from sulphide copper minerals in which an ore pulp is conditioned with an alkaline reagent followed by a second conditioning step in the presence of a collector. A surface modifying agent is then added to the conditioned ore pulp at a rate and in an amount sufficient to decrease the platinum electrode potential of the pulp, when measured against a standard colomel electrode, to a minimum of -100 mV. The ore pulp is then conditioned for a time sufficient to achieve intimate contact between the ore particles, and thereafter subjecting the so treated ore to conventional flotation.
Fleming (op cit) mentions a treatment scheme for an ore containing cerussite, vanadinite and descloizite involving the formulation of a pulp of these minerals and treating them with a sulphidisation agent for flotation.